Rosacea is a common, chronic and progressive inflammatory disease with skin features characterized by blushing and flushing, facial erythema, papules, pustules, telangiectasia and sometimes ocular lesions known as ocular rosacea. In severe cases, particularly in men, rhinophyma, or a bulbous enlargement of the nose, may occur. Rosacea develops over the course of several years with periods of exacerbation triggered by various stimuli such as temperature changes, alcohol, spicy foods, sun exposure and emotional factors.
The prevalence of rosacea in the European population ranges between 0.09 and 22%, with a peak age of onset between 25 and 70 and is much more common in people with a light complexion. It more particularly affects women although the condition is generally more severe in men. The prevalence of family histories of rosacea has been reported.
Four subtypes of rosacea have been defined according to the degree of primary features, such as vasomotor flushing, persistent erythema, papules and pustules, telangiectasias (Wilkin J et al., JAAD, 2002, 46: 584-587):                Erythematotelangiectatic rosacea (ERT) is mainly characterized by vasomotor flushing and persistent central facial erythema. Telangiectasias are commonly observed but are not essential for the diagnosis of this subtype. Central facial edema, burning or stinging sensations and rough, flaky skin are also symptoms that have sometimes been reported. A history of flushing as the only symptom is commonly found in people with erythematotelangiectatic rosacea.        Papulopustular rosacea (PPR) is characterized by persistent central facial erythema and transient crops of papules and/or pustules in the center of the face. However, the papules and pustules can also occur in periorificial regions, i.e., around the mouth, nose and eyes. The papulopustular subtype resembles acne vulgaris, but comedones are absent. Rosacea and acne may coexist in a same patient, in which case comedones may also be present alongside the papules and pustules suggestive of rosacea. People with papulopustular rosacea sometimes complain of a burning or stinging sensation. This subtype is often observed before or at the same time as ER (including the presence of telangiectasias). The telangiectasias may be obscured by the persistent erythema and the papules and pustules, but they tend to become more visible after successful treatments that cover up these features.        Phymatous rosacea is characterized by a thickening of the skin, irregular surface nodularities and swelling. The nose is most commonly affected but phymatous rosacea can also involve other areas such as the chin, the forehead, the cheeks and the ears. Patients with this subtype sometimes exhibit prominent, enlarged follicles in the affected areas as well as telangiectasias. This subtype often occurs before or at the same time as ER or PPR (including the presence of persistent erythema, telangiectasias, papules and pustules). In the case of rhinophyma, these additional stigmata may be particularly pronounced in the nasal region.        Ocular rosacea (or ophthalmic rosacea) exhibits symptoms restricted to the ocular area with blepharitis, conjunctivitis and keratitis. The diagnosis of ocular rosacea should be considered when a patient presents with one or more of the following ocular signs and symptoms: watery or bloodshot eyes (interpalpebral conjunctival hyperemia), foreign body sensation, burning or stinging, dry or itchy eyes, sensitivity to light, blurred vision, conjunctival telangiectasias or eyelid margin telangiectasias or erythema of the eyelid and periocular area.        
The pathogenesis of this disease is still unknown and might differ according to subtypes. Many studies describe rosacea as a vascular system disorder related to UV exposure or an immune disorder with an increased expression of pattern recognition receptors, which triggers an exacerbated response to microorganisms such as Demodex mites.
The Demodex mites have been studied in rosacea patients to investigate their role in the pathogenesis of the disease. However, the importance of these mites remains controversial. Indeed, a small number of studies failed to demonstrate an increase in Demodex density in patients with rosacea. Moreover, culture-dependent studies do not allow for a consistent picture of the skin microbiota to be obtained.
Surprisingly, by conducting experiments and researches on Demodex mites, the inventors have demonstrated that the microbiota of Demodex mites differs between rosacea patients and control. Furthermore, the inventors have shown that the characterization of the specific microbiota associated with Demodex may be performed using a method of 16SrRNA sequencing. These results can be advantageously used for implementing a method for the diagnosis of rosacea.